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- 1 Move entry to Gas constant
- 2 Specific gas constant
- 3 Answering Seth Ilys
- 4 Answering 126.96.36.199
- 5 R *is* the Boltzmann Constant
- 6 2/3 cut - ref please
- 7 R=2 (2/1), R=1 (2/2) or R=2/3?
- 8 Merge specific gas constant
- 9 8.314472 or 8.314482?
- 10 Reversion of edit from 188.8.131.52
- 11 R = 8.314472(15) J · K-1 · mol-1
- 12 Added a note on three of the constants in the table
- 13 Too many units
- 14 Entropy and R: units
- 15 Numeric representation - the parentheses are confusing
- 16 Energy per kelvin per mole
- 17 Rbar
- 18 Constant Verification
- 19 Which calories?
- 20 Mendeleev
- 21 Very helpful
Move entry to Gas constant
This entry should be moved to Gas constant or even Universal gas constant and it is the Molar gas constant that should be REDIR-ed. Whatever you do with the units etc. it does not change the sense - is there any other (universal) gas constant? Its sense lies in its molar nature and you do not need to do is again (buttery butter). If it is not molar, then it has its own name and it is Boltzmann constant, k (same but per molecule), and R = NAk. If you need sth more like specific heat (J /g K)it stops being constant ...
I know that some sources use this molar adjective - even Encycl. Britannica - but if you check sth more serious like P.W. Atkins "Physical Chemistry" (4th ed., Oxford 2000), IUPAC's Goldbook http://goldbook.iupac.org/goldbook/G02579.html , Gas constant is all you get and it is all you need.
AWM~ads 21:47, 11 Mar 2005 (UTC)
- An IUPAC convention is most certainly a convincing argument. However, another serious source CODATA does use the molar adjective. See also "CODATA recommended values of the fundamental physical constants, 1998", Rev. Mod. Phys., vol. 72, No. 2, 2000. So, the issue seems debatable and at least not very urgent.
- That being said, your arguments are sound and IUPAC deals more with naming conventions than CODATA. Therefore, I support the move.
- Jan van Male 09:36, 12 Mar 2005 (UTC)
- I like your attitude. I suggest similar move for Avogadro's number which is not a number but an Avogadro's constant. AWM~ads 22:50, 25 Mar 2005 (UTC)
- I don't think a similar name for Avagadro's number would be fitting, as in my experience someone involved in the field is more likely to call it Avagadro's Number than Avagadro's constant. On the other hand, the gas constant does simply get called the gas constant, with some adjective infront of gas.EagleFalconn 19:11, 22 Apr 2005 (UTC)
Specific gas constant
This page (gas constant) should not talk about the universal gas constant exclusively. Dividing the universal gas constant by the molar mass of a gas results in a new gas constant (specific to the gas). There was already some confusion on speed of sound where the text linked to gas constant explicitely claiming that was in fact the universal gas constant which it clearly isn't.
Answering Seth Ilys
The article said: The Boltzmann constant is conversion factor between gas units. It tells how many joules per kelvin make a molecule. Seth Ilys deletes it saying: I have no idea what that sentence actually means, let alone what it's supposed to mean. Well, if N is the number of molecules then Nk=PV/T is the amount of gas measured in joule per kelvin, so k is the amount of gas of a single molecule. Bo Jacoby 05:12, 31 January 2006 (UTC)
You write: Because the gas laws are so inexact for real gases, the gas constant is not known as accurately as most other fundamental constants of physics. This is not true. 6 significant digits is not bad, and in the low pressure limit the ideal gas law is precise. Bo Jacoby 09:07, 2 February 2006 (UTC)
- Agreed. 6 significant digits seems pretty good compared to other constants. The gravitational constant has fewer. I wouldn't mind having list of physical constants, though, Category:Physical constants is not a great substitute. Algae 09:42, 2 February 2006 (UTC)
R *is* the Boltzmann Constant
for the same reason. It's just usually expressed in liter-atmospheres (a unit of energy) per degree Kelvin (a unit of temperature) mole (unitless), rather than in Joules per Kelvin. Check it yourself -- when you convert the units, 0.08206 liter atmospheres per kelvin mole is 1.38e-23 J/K.
The reason for the similarity is that the pressure of an ideal monatomic gas is the same as its internal energy density, according to the kinetic theory of gas.
So, er, I'm sticking Boltzmann constant back in (it was recently diked out) zowie 15:18, 31 March 2006 (UTC)
2/3 cut - ref please
Removed the following:
- If we define the temperature of a gas as the average kinetic energy of its molecules then the constant simplifies further when the units are standardized. Instead of measuring energy sometimes in Kelvin and sometimes in Joules, the same unit could be used for both measurements. Instead of the gas constant being in Joules per Kelvin, the constant would then be dimensionless and reflect simply the physics of the gas law and not the conversion between different units. The true value of the gas constant can then be seen to be simply 2/3.
- PV represents a double counting of the proportion of kinetic energy in a gas in one direction, eg up-down, left-right, or backwards-forwards. The double counting comes from calculating m*v*v whereas kinetic energy is 1/2*m*v*v. If molecules hit something and stop, one would be measuring 1/2*m*v*v. If they hit something and bounce back, then we are measuring m*v*v.
- nT represents the total kinetic energy in a gas in all three dimensions and not just perpendicular to a plane. 1/3 of the energy is in each direction, up-down, left-right, or backwards-forwards.
- PV/2 = nT/3
- The Universal gas constant therefore has a value of 2/3.
- Physicists usually don't explain that as they don't really understand it themselves.
- Ivan Urwin
Provide a ref. Ivan or it stays out :-) Vsmith 18:03, 6 May 2006 (UTC)
What do you have a problem with? RHS: nT/3 - one third total kinetic energy - that only a third of the kinetic energy would be perpendicular to a given plane? LHS: PV/2 - Or that the m*v*v in pressure calculations double counts kinetic energy of 1/2*m*v*v perpendicular to a plane? Moles are dimensionless, just like a dozen is 12. You just need a calulator and to convert between Kelvin and Joules to verify it. You could call the gas constant 8 inchs per foot if you want! Of course if physicists want to put it in ergs per dozen electron volts or BTUs per mega barrel of oil equivalent or Joules per mole Kelvin instead of getting their calculators out: fine! That won't stop the fundamental physics of the gas equation saying its 2/3.
Here, not rigorous physics, but enough that people can see for themselves that the gas constant is 2/3 when coherent units for energy are used.
Take a particle of mass m going at velocity v, and to simplfy calculations imagine it in a sphere of radius v. Time taken from centre to perimeter is then 1 second, and time back to the centre is another second. Exchange of momentum is then 2mv in 2 seconds, and rate of change of momentum is what some guy called Newton called force. So the force is mv, and the pressure on the sphere is mv/A where A is the area. Volume in three dimensions is 1/3*base*height, so the volume of the sphere is (1/3)*v*A.
PV = (mv/A) * (1/3)*v*A = (1/3)*m*v*v = (2/3)*(1/2)*m*v*v = (2/3)*T
Sum over n particles ... The gas law is PV = n*(2/3)*T Call the constant R. R=2/3. Change the units of measurement 2/3 is 4.01E26/kmol (in the same way that 2/3 is 67 per cent) 2/3 is then 8310 J/Kelvin/kmol (in the same way that 2/3 is 8 inches per foot) R=8310 J/Kelvin/kmol
- OK, we have a particle with velocity v bouncing around in a sphere of radius v to simplify calculations...and this is published in ??? Vsmith 00:19, 7 May 2006 (UTC)
The constant only has a different value if you measure the heat applied in Joules or BTUs and the temperature rise in Fahrenheit or Kelvin, etc. The mental problem people have is in switching between units: You can measure a tank's fuel efficiency in gallons per mile and the area of a field in acres. Somebody that understands what they are doing can cope with measuing areas in gallons per mile, or measuing a tank's fuel efficiency in acres or square metres. For some people tax is 20 cents in the dollar. Other people manage to cancel the two different units of currency and just specify the tax rate as a dimensionless 1/5.
Take a monatomic gas and heat it. All (ie. 100%, 1) of the heat causes a temperature change, rather than causing the molecules to spin or vibrate.
3/2R = 1.
Do I have a reference for 3/2R when heating monatomic gases? No!
R=2 (2/1), R=1 (2/2) or R=2/3?
Anybody who has seen Newton's cradle - the swinging balls on strings - will know that if a particle hit another and transferred all its kinetic energy, then it comes to a stop. This is obvious anyway, without any kinetic energy, something cannot be moving. When we measure PV, (pressure times volume,) we are looking at a state where molecules collide with the surface they exert a pressure on, and bounce back. We are therefore measuring something based on twice (2!) the kinetic energy perpendicular to the surface. Assuming the kinetic energy is equally partitioned in three dimensions, that is equivalent to saying that PV measures the translational kinetic energy in two dimensions, since twice the energy in one ddimension equals the energy in two dimensions.
Bolzmann's constant (k) in the gas law does two things. It partly converts units, between joules and Kelvin say (for example) and it partly fills the role of the gas constant, which has a value telling us that PV is measuring the kinetic energy of the gas in two of the three dimensions of space, or equivalently, twice the portion of kinetic energy in one dimension, namely twice that perpendicular to the surface the pressure acts on.
This means that we can convert between temperature energy and other energy, kelvin and joules (for example), but only once we decide on a definition for temperature. Different definitions of temperature will result in different values of the gas constant and different values of the conversion constant. If I use strange units to measure circles, then you can calculate and convert satifactorily once you measure a circle for yourself and compare with my figures. However the 'universal' constant you use includes no knowledge of whether I am measuring a radius or a diameter. Only when you understand what my measurement really means can you determine a proper constant value for converting units, and assign another constant to account for measuring different features of the circle, eg radius, diameter or even circumference.
If I measure diameter and you measure radius, then a factor of two in your universal constant will result from the physics of what is going on, and the rest will be units conversion. The conversion factor might be so many weirdo units per dozen inches. Somebody else might think 'dozen inches' is a bit strange and come up with a different constant in weirdo units per inch. This is what has happened with the gas constant in joules per mol kelvin, or joules per kelvin.
(A) If we define temperature as the energy per degree of freedom, (the kinetic energy in one dimension) then the gas constant is R=2 and the conversion between joules and kelvin is with a constant k/2.
(B) If we define temperature as twice the energy per degree of freedom, then the gas constant is 1 and the conversion between joules and kelvin is with a constant k. This I believe is an unnatural choice. It is however what happens when people take the Bolzmann constant as being simply a conversion factor and forget about the gas law.
(C) If we define temperature as three times the one dimensional kinetic energy, (ie the kinetic energy in three dimensions - the total translational kinetic energy), then the gas constant is 2/3 and conversion between joules and kelvin is with a constant 3k/2.
The first choice (A) is not a bad choice to make. For example we read for diatomic gases around room temerature that Cv~=(5/2)*R. With R=2 this becomes Cv=5. This is obvious; the total energy required to heat something with energy equally split between 5 degrees of freedom is simply five times the energy required per degree of freedom.
Poor students are then being confused by people teaching this stuff that do not understand it themselves. To elimintate the conversion constants it becomes necessary to take a ratio of two values that both include the conversion constant, eg a Cp and a Cv, and call this gamma; and then, to manipulate this to come up with stuff like (gamma-1)/gamma to get to something which should be fundamentally simple to start with.
Take a diatomic gas around room temperture and use definition A for temperature. Cv ~= 5, Cp~=7, since (PV stuff again) it takes the energy equivalent of a couple of degrees of freedom to push back the environment.
Gamma = Cp/Cv = 7/5
Gamma-1 = 2/5
(Gamma-1) / gamma = (2/5)/(7/5) = 2/7.
We are back to a simple ratio of counting a few degrees of freedom, and the clarity of this is completely remove from the students mind by doing calculations in joules per mole kelvin with figures only suitable for calculators, etc.
Merge specific gas constant
It will be useful to merge the article on specific gas constant here. I guess many readers will be interested in knowing about both the terms and it would be better to have them on one page. -Myth (Talk) 17:54, 27 February 2007 (UTC)
- I second that. A merge would useful. Verkhovensky 05:20, 28 February 2007 (UTC)
8.314472 or 8.314482?
Slight inconsistancy in the statement of the value of the Universal Gas Constant. In the table it says 8.314482. I assume this is just a misspelling but I'm not confident enough to correct it myself so thought I'd make sure.
184.108.40.206 16:28, 17 April 2007 (UTC)
This is my first post on this page: 8.314472(15) J K−1 mol−1 is the only unit from reference #1, the rest are derived mathematically, perhaps the note should be moved to reflect this.
Secondly the units in atmospheres are derived from the joule unit above by the relation 101325 Pa = 1 atm exactly. Thus these units are correct: 8.205746×10−5 m3 atm K−1 mol−1, 82.05746 cm3 atm K−1 mol−1, This unit is wrong: 0.0821(14) L atm K−1 mol−1, Maybe it is from an earlier source or there was a math error, but it should be fixed. the correct value is: 0.08205746(02) cm3 atm K−1 mol−1, The (02) could also be added to the other atm units.Rgbutler (talk) 14:22, 16 April 2010 (UTC)
- Check some of the values displayed on the page. In the main text 8.3154621(75) is nowhere close!
Reversion of edit from 220.127.116.11
I went and reversed a change made by 18.104.22.168. It had been changed from the proper value in terms of (L*atm)/(K*mol) (0.0820574587) to 82.0574587, which is the same value, but in terms of mL. Liter is the standard measure for gases, so that (0.082...) is the proper figure to keep.
R = 8.314472(15) J · K-1 · mol-1
Added a note on three of the constants in the table
Three of the values for the universal gas constant contained in the table are for air only. Since these three are mass-based vs. molar based constants, they can only describe a substance with a certain molar weight, which is in this case air. I added a note mentioning this, as there did not seem to be anything already in the article noting this fact. —Preceding unsigned comment added by 22.214.171.124 (talk) 03:35, 7 April 2008 (UTC)
Too many units
Do we really need that huge table of the gas constant in different units? For example, who uses slugs? Why are there so many versions that seem to be in terms of mass? I think we need to trim the list a bit. Mathfan (talk) 17:56, 2 May 2008 (UTC)
Mathfan: don't be an elitist. For reasons you may not understand, Engineers use several dozen sets of units. Why the fuck did you get rid of that table? That table has helped me for the past few months and now I'm having trouble finding R at the esoteric units that I need.
the table with many units is very useful as a quick reference and in no way detracts from the quality of the article. —Preceding unsigned comment added by 126.96.36.199 (talk) 20:36, 24 October 2009 (UTC)
I also do not understand why we should delete values in a reference table just because the SI-nazis frown upon them. Welcome to the real world, where people in different professions use different units. -188.8.131.52 (talk) 02:11, 9 April 2010 (UTC)
I am concerned about the table simply because I don't know the source of most of the data. If you check the given reference, it cites only the very first value in the table. All those other table entries are coming from other unknown sources. I have no idea whether they are reliable! —Preceding unsigned comment added by Dcbrc2 (talk • contribs) 20:39, 10 August 2010 (UTC)
Do not delete the table. When referencing old engineering journals, despite my displeasure, they often use these units. —Preceding unsigned comment added by 184.108.40.206 (talk) 20:32, 14 January 2011 (UTC)
Entropy and R: units
I just wanted to ask if anyone knew actually why the units are the same. Why is R a measure of entropy?
I don't know if this is just a stupid question or not, but my chemistry teacher didn't know and asked me, and said to tell her when I found out.
I have a keen interest in chemistry and physics and stuff, so don't worry about it being over my head!
Numeric representation - the parentheses are confusing
The article says R=8.314 472(15) SI units. It turns out the 15 in parentheses is actually the standard deviation in the last two digits.
Parentheses in math usually denote something else completely. The usual meaning of 8.314 472(15) would be 8.314 472 times 15.
This use is non-standard. It is highly confusing. Can it be replaced with a more standard notation?
- Erm, it is standard notation for representing the standard uncertainty in the least significant figures. Physchim62 (talk) 00:34, 28 October 2009 (UTC)
Decimal Places or Multiplication
The use of (15) after the constants is not universally know. I assume its number of decimals. The parantheses could be confused with multiplication. Maybe we want to consider a more common way of representing ongoing digits. —Preceding unsigned comment added by Russot1 (talk • contribs) 02:33, 12 February 2010 (UTC)
I don't understand it, and it sounds like you weren't the one who made it. If we could get something I've been taught is a standard (Confidence interval, significance level), that'd be great. Once I understand it I'll try and edit it. Someone seems to have just copied it from a huge table with a special notation to save space. Inthend9 (talk) 18:57, 1 March 2010 (UTC)
- It seems fine to me. That is how it's presented in the reference, and the article explains what it means. After a quick search through WP, it seems that most other physical constants are presented the same way. Djd sd (talk) 06:41, 4 March 2010 (UTC)
The numbers in parentheses are the additional decimal points which have been measured but lie outside the accuracy of the measurement process. This means the values in the parentheses eg (15) could be used for the purpose of calculation, but any value that results from such a calculation would not be more accurate than the number of significant figures if those in parentheses were not considered. In other words, the values in parentheses are there to help avoid rounding errors if you’re trying to work to the limit of accuracy of the gas constant. I agree that this is not commonly known, and only confuses the issue. I would recommend removing it. —Preceding unsigned comment added by 220.127.116.11 (talk) 13:08, 25 May 2010 (UTC)
Energy per kelvin per mole
The intro said that R is energy per kelvin per mole, but that's inconsistent - energy is a quantity, and kelvin is a unit. I changed it to energy per temperature increment per mole; I considered energy per temperature per mole, but that sounded awkward to me. As for mole, I left it in as is, because it's the only thing that distinguishes R from k. (Related discussion here: http://en.wikipedia.org/wiki/Talk:Ideal_gas_law#n_.3D_number_of_moles.3F ) Khakiandmauve (talk) 16:25, 8 March 2011 (UTC)
User:18.104.22.168's edit to the equations in the specific gas constant section suggests that Rbar is used for the specific gas constant. I haven't seen that usage. I'm reverting the edit, as I see the equations as distinguishing the specific gas constant from the universal constant. Different notation conventions are discussed in the text below. If anybody is familiar with the use of Rbar for specific gas constant, I request that you add it there, preferably with a source. Khakiandmauve (talk) 19:51, 30 March 2011 (UTC)
My first edit on Wikipedia, so I thought I would give it some background.
The specific gas constant of air, Rair, provided in the Society of Flight Test Engineers Reference Handbook - 2007 (page 01-43) is given in (ft*lb)/(lbm*degR) - degR = degree Rankine. A more convenient unit mix for my application was (ft*lb)/(slub*degR). The calculation: Rair = 53.35 (ft*lb)/(lbm*degR) = 1716.49 (ft*lb)/(lbm*degR).
Therefore I changed the specific gas constant given in the table from 1716.59 (ft*lb)/(lbm*degR) to 1716.49 (ft*lb)/(lbm*degR). A small change, but it was significant in my calculations. — Preceding unsigned comment added by Dbruce.ae05 (talk • contribs) 19:15, 22 November 2011 (UTC)
The table entries in calories don't say which kind of calorie they are using. It seems that it is the International Steam Table calorie, which is equal to 4.1868 J. But everybody knows that the One True Calorie is the thermochemical calorie, which is 4.184 J! :-) --Itub (talk) 18:46, 26 October 2012 (UTC)
- I'll remember that when next I'm standing in a buffet line, scooping my calories up off a steam table. They're a little bigger than I thought... To bad the difference doesn't explain the problem the user above has, since it's too large by an order of magnitude to explain the difference between 1716.59 and 1716.49. SBHarris 01:18, 27 October 2012 (UTC)
I'm a chemical engineer who practices in Europe and the USA. Many journals use a "international" Joule based upon a thermochemical calorie. Thus the gas constant for most energy calculations is 8.3130 instead of the 8.3144. See for example page 1-18 Perry's Chemical Engineer's Handbook, 7th Edition. This allows for valid energy balance calculations in engineering. There should be some discussion of this here. — Preceding unsigned comment added by 22.214.171.124 (talk) 21:39, 4 December 2013 (UTC)
I don’t read Russian, but I take it the references User:Watergrain adduced in support of the term “Mendeleev’s constant” show that Dmitri Mendeleev did important or pioneering work in defining or measuring the constant we now know as R. That’s all well and good, but the man’s own work can’t possibly support the claim that R is named after him in the literature, which is what “known as“ implies. Unless secondary sources—which in this case could include other scientists’ research papers–can be found to show that the term has any currency, or even historical presence in writing about the topic, its mention should be removed as WP:OR. There may be something worth summarizing in those references, however, to go elsewhere in the article. Any Russian-speakers who can summarize the relevant parts here?—Odysseus1479 05:26, 19 May 2013 (UTC)
For what it's worth, the section "dimensional analysis" clarified for me the meaning of the ideal gas law. PV is simply some molecules at some temperature (average kinetic energy), and the gas constant is just a tool for calculation. I don't know what the average non-expert reader wants from an article - but this is exactly what I look for. ---lifeform (talk) 23:02, 24 October 2014 (UTC)